There are numerous applications in which it is necessary or desirable to control some safety interlock or other device in accordance with the speed of a rotating shaft. Vehicular applications are common; for example, a control to latch or close the doors of a passenger vehicle whenever the vehicle is moving above a predetermined threshold speed, or an electrically actuated interlock mechanism to preclude shifting the transmission into reverse whenever the vehicle is moving forward at even a limited threshold speed, or vice versa. Similar speed switch control needs, based upon the rotational speed of a shaft or like rotary member, are also commonly encountered in machine tools and other industrial equipment.
The requirements imposed upon speed switch controls, particularly those employed in vehicular applications, are frequently quite severe. Thus, the control may be subjected to high levels of vibration and to substantial shock forces. Electrical transients of substantial magnitude may be encountered. Because the control does not perform a primary operational function, it is frequently subject to severe cost limitations. In addition, it is highly desirable that the electrical connections to the control be as simple as possible, preferably constituting a simple two-terminal connection, to minimize cost and to facilitate replacement when necessary.
A variety of different speed switch controls have been devised for use in applications of this kind; many start with an input signal derived from a small AC generator driven by the shaft or like rotary member being monitored. The circuits of these devices have often been undesirably complex and costly, particularly when operation is based upon the frequency of the AC input signal, requiring a frequency/voltage conversion stage as a part of the control circuitry. These controls are difficult to construct in a form rugged enough for vehicular applications, in large part due to the circuit complexities introduced by frequency/voltage conversion. Moreover, many of these speed switch controls require three or more terminal connections. These problems are particularly acute in speed controls applied to vehicles.
Inexpensive precision two-terminal speed switch controls are described in the aforementioned co-pending United States applications of M.A. Lace, Ser. Nos. 732,332 (now U.S. Pat. No. 4,086,647) and 745,453 (now abandoned). But those controls are not satisfactory for critical speeds below ten revolutions per minute, where the output amplitude of the AC generator is quite small. This is particularly true in passenger vehicle safety control applications, where a shaft speed of two rpm or even less may constitute the critical speed.
Moreover, a number of other important operating characteristics have been difficult and sometimes impossible to realize with previously known speed switch controls. Thus, it is highly desirable to provide a single basic speed switch control circuit that can be readily converted from operation as a normally open switch to operation as a normally closed switch, and vice versa, to meet the varying requirements of different safety devices and other loads. It is equally desirable to have a single basic speed switch control circuit that is capable of operation over a broad range of critical rotational speeds, from nearly zero rpm to hundreds of revolutions per minute, to minimize custom design of circuits to fit individual applications. Another critical requirement, in many applications, is the limination of "hunting" when the rotational speed of the input shaft is subject to substantial variation over a brief period of time. In addition, a practical and effective precision speed switch control should require only a low power drain but should be capable of handling relatively high currents, so that it can be readily adapted to a variety of different specific applications.